1. Field of the Invention
The invention relates to a device and a method for processing cardiac signals, in particular an electromedical implant, e.g., a cardiac pacemaker, a cardioverter/defibrillator or the like as well as a method for operating such a device.
2. Description of the Related Art
Such implants are used in an essentially known manner to support or ensure adequate contraction of the chambers of the heart (ventricle and/or atrium) over time. Such electromedical implants are usually designed to detect natural contractions of the respective chamber by and evaluating the electric potential of the myocardium of the respective chamber associated with the natural contractions. The chart of these potentials is also known as an electrocardiogram, which is recorded between two electrically conductive electrodes, suitably arranged on or near the myocardium. Various types of electrocardiograms can be differentiated, depending on how and where the potential charts are plotted. If potential charts are detected by using a plurality of electrodes attached to the skin of the human body, this is called a superficial electrocardiogram. If the potential charts are detected with the help of an electronic implant and with the assistance of intracardiac electrodes connected to the implant, this is called an intracardiac electrogram. Of the intracardiac electrograms, near-field electrocardiograms can be differentiated from far-field electrocardiograms, depending on how far away the electrode recording the potential is from the site of excitation. The intracardiac detection may be accomplished in a bipolar manner, i.e., between two electrode contacts of an electrode line placed in the atrium or ventricle. As an alternative or in parallel with this, unipolar detection is also possible, i.e., between an electrode placed in the atrium or ventricle and a suitable mating electrode formed by the electrically conductive housing.
By recording electrocardiograms, natural (and also stimulated) excitation of the respective myocardium can be detected in an essentially known way. For example, the excitation of both ventricles is manifested in a strong signal segment of the respective electrocardiogram, known as the QRS complex. Excitation of the atrium is detected, for example, by detection of so-called P waves in the respective electrocardiogram. In this way, important events including the time of occurrence can be detected for control of an electromedical implant, in particular an implantable electrostimulator. Such events may be natural or stimulated atrial events, i.e., natural or stimulated excitation of the atrium resulting in a contraction of the atrium. Accordingly, the detected events may also be natural or stimulated events in the respective ventricle (right or left ventricle), i.e., natural or stimulated excitation of the respective ventricle with subsequent contraction.
Firstly, events thus detected in the heart are used to start certain timers in an implantable electrostimulator but also to detect certain intervals of time and to use them to control the electrostimulator. For example, by detecting a right-atrial natural event, (i.e., excitation of the right atrium), a timer may be started for a right-ventricular escape interval, at the end of which a right-ventricular stimulation pulse is delivered if a natural contraction of the right ventricle is not detected before the end of the right-ventricular escape interval. With so-called demand pacemakers, delivery of a ventricular stimulus is suppressed when a natural event is detected in the chamber that is otherwise to be stimulated during the course of a corresponding escape interval.
To adapt such a right-ventricular escape interval as accurately as possible to the needs of the individual heart, it is advantageous if the right-ventricular escape interval is adapted to a natural atrioventricular conduction time (AV delay) from the right atrium to the right ventricle and, as a rule, is only slightly longer than this AV interval.
It is likewise desirable to determine a suitable interval for the time lag between excitation of the left atrium and that of the left ventricle. In programming this time lag, it should be noted that active contraction of the left ventricle should occur approx. 100 ms after the left-atrial electric excitation (end of the left-atrial P wave), because the filling phase of the left ventricle is concluded only after approx. 100 ms. The start of the left ventricle contraction correlates with total excitation of the ventricular myocardium, as illustrated by a peak in the QRS complex in the superficial electrocardiogram.
With an electromedical implant for stimulation of the left ventricle, the corresponding AV time of the implant should also control mechanical synchronization between the left atrium and the left ventricle, so that contraction of the left ventricle does not begin before conclusion of the left-atrial ejection phase. The duration of the left-atrial ejection phase and/or the active filling phase of the left ventricle may be assumed to be a constant empirically determined value.
Electrodes in the right atrium and/or right ventricle are usually used to control the electromedical implant. For detection of atrial events with this so-called right-atrial electrode position, there is a conduction time between detection of the right-atrial natural event and/or right-atrial stimulation and the onset of the left-atrial ejection phase. However, the duration of this conduction time cannot usually be determined. The same thing is also true of the conduction time between a right-ventricular stimulus and the onset of the left-ventricular contraction, when ventricular events are detected with this so-called right-ventricular electrode position.
The latency times, which depend on the conduction times, are subject to interindividual variations and must therefore be measured individually on each patient.